They have prescribed you an antibiotic they think is safe - this will be down to clinical experience, as it's unethical to do trials on pregnant women.
Reading around, there have been no reports on adverse outcomes of using flucloxillin in pregnancy and it's one of the antibiotics of choice for pregnant women
Not taking an antibiotic when you need one, however can lead to problems for you, the pregnancy, and the baby.
Untreated infections can lead to early labour, septicaemia and all kinds of nasty things.
Starting and stopping antibiotics before the end of the course can lead to resistance, not a good idea either.
Lancing the boil yourself sounds like a really bad idea to me, too.
Why do you think it would make any difference if you go into labour? Women with GBS for example are purposely on antibiotic drip during labour to protect the child.
I'm on antibiotics myself at the time being, started off with IV about 3 weeks ago, had two week long courses, and because that still hasn't finished the nasties off, will stay on them until the baby is born.
Anyway, please listen to your doctors and take the prescribed medication. Don't go by gut feeling and strangers of the internet (unless the strangers tell you to listen to the doctors)